It doesn’t make sense to compare these numbers to the Israel Day parade, because there are far more “MO” and mostly/completely non-observant Jews than there are observant Jews.
So given that additional pool of people (secular Jews), it makes sense that a gathering of all frum Jews will not come close to a gathering of secular (and “D”L”) Jews.
As well, unfortunately, for many Jews, the two gatherings are not mutually exclusive. I imagine there were Jews who attended Sunday’s gathering but will still not understand why it makes no sense to attend the Israel Day parade.
Whether 50K or 150K, and on very limited notice, this is quite a large number of observant Jews who understand the evil perpetrated by the Zionists on our brethren in E”Y, B”H.